1. Field of the Invention
This invention relates generally to molecular beam epitaxy, and more particularly to improvements in a molecular beam epitaxy apparatus.
2. Description of the Prior Art
Molecular beam epitaxy (often abbreviated to "MBE") has attracted attentions as a method for epitaxially growing a thin layer on a monocrystal substrate in the production of compound semiconductors, particularly group III-V compound semiconductors. In MBE, which is one of the vacuum deposition methods, group III elements such as Ga, Al, In and group V elements such as As, P are emitted in the form of molecular or atomic beams (As.sub.2 or As.sub.4 in the case of arsenic) under a ultra high vacuum of e.g. 10.sup.-11 Torr, and deposited on a monocrystal substrate of e.g. GaAs, InP to form an epitaxial layer or layers of e.g. GaAs, AlGaAs, InP, InGaAsP.
The MBE is known to have various advantages. Several of these advantages are as follows:
(1) Due to the use of a ultra high vacuum, it is possible to always keep clean the growth front of each substrate by expelling gaseous impurities, consequently improving the product quality. PA1 (2) Because of high vacuum, it is possible to deposit a uniform layer or film over a large area. PA1 (3) It is possible to precisely control the film thickness in angstroms because the crystal growth rate can be made very low and accurately adjusted. PA1 (4) It is possible to readily obtain a thin film of multi-component mixed crystals by simply increasing molecular beam sources. PA1 (5) Molecular beams used for crystal formation can be also used to detect the surface or growth conditions during crystal growth, so that useful information can be immediately fed back for controlling the crystal growth.
For further understanding of the MBE technique, reference is now made to FIGS. 14 and 15 of the accompanying drawings which show a typical conventional MBE apparatus.
As shown in FIG. 14, the prior art MBE apparatus comprises a growth chamber 102 connected to a high vacuum pump 101 for evacuating the chamber interior to a ultra high vacuum. Centrally within the chamber 102 is a holder support 103 which is controllably rotated about a central vertical axis L. The support 103 receives a substrate holder H' loaded with a plurality of substrates B' (see FIG. 15). Rotation of the holder support is required to ensure uniform crystal deposition on every substrate B'. The support 103 is provided with a heater (not shown) to heat the substrate to a temperature suitable for crystal growth.
The growth chamber 102 is provided at its bottom with a plurality (only two shown) of molecular beam sources or vaporizers 105 disposed in an annular arrangement around the central vertical axis L of the growth chamber for generating molecular beams of different materials. Each source 105 is substantially equally spaced from the substrate holder H', and has a longitudinal axis A directed to the center of the holder. The source 105 includes a crucible 104 for receiving a suitable material which is heated by a heater (not shown) for vaporization. The source 105 further includes a shroud or cool trap 106 to which is supplied liquid nitrogen for preventing the source from being thermally influenced by the other sources. The molecular beam emission is controlled by opening and closing a shutter 107 arranged at the emission opening of the vaporizer 105.
For depositing a GaAs layer on GaAs monocrystal substrates for example with the above apparatus, two of the vaporizers 105 for Ga and As respectively are heated while the substrate H' with the substrates B' is also heated to a suitable temperature, and thereafter the relevant shutters 107 are opened for a predetermined period. An additional element Al may be simultaneously vaporized to form a Ga.sub.x Al.sub.1-x As layer in which the value of x is determined by the ratio in vaporized amount between Ga and Al. Further, simultaneous vaporization of Si or Sn provides a n-type crystal layer, whereas simultaneous inclusion of Be or Mg provides a p-type crystal layer.
As shown in FIG. 15, the MBE apparatus further comprises a preparation chamber 110 connected to the growth chamber 102 via a first gate valve 108. The preparation chamber 110 is provided with a second gate valve 109 for hermetically separating the preparation chamber from the atmosphere.
For feeding a new set of substrates, the first gate valve 108 is held closed while the second gate valve 109 is opened, and the set of substrates B' retained by the substrate holder H' is supplied from outside into the preparation chamber 110 through the open second gate valve 109. Subsequently, the second gate valve is closed, and the preparation chamber is evacuated to a high vacuum, whereupon the first gate valve 108 is opened to supply the substrate set into the growth chamber. In this way, the time required for re-evacuating the growth chamber 102 to a ultra high vacuum is greatly reduced, thereby increasing the productivity of the apparatus.
In the absence of the preparation chamber, obviously, vacuum leakage upon feeding a new set of substrates makes it necessary to re-evacuate the growth chamber from the very start, consequently requiring a long time. It is for this reason that the MBE has been formerly evaluated only as an experimental or research tool.
The arrangement shown in FIG. 15 opens up (gives at least a hint to) the way for application of the MBE on commercial scale. However, such an arrangement still has the following disadvantage.
According to the arrangement of FIG. 15, not only the set of substrates B' but also the holder H' therefor must be supplied from the exterior. When brought into the growth chamber 102, the holder, which is usually made of molybdenum or a molybdenum alloy to ensure high strength, stability at high temperature and corrosion resistance, will allow simultaneous entry of adhered moisture content, air or other contaminants into the growth chamber. Because the requirements for the interior condition of the growth chamber during crystal growth are very stringent, it is difficult or time-taking to acceptably expel the adhered contaminants by evacuation.